The present invention relates to a process for the concentration of alkaline solution wherein the alkaline solution is concentrated under atmospheric pressure in the form of a heated falling-film to form a practically anhydrous melt. The present invention is also directed to an apparatus for conducting the present process.
NaOH and KOH solutions are obtained, for example, in chlorine manufacture by NaCl electrolysis, as an 11-50% solution. It is known to concentrate such solutions in falling-film evaporators to a practically anhydrous melt, and to pour the melt subsequently into barrels or to further process them into flakes or prills. Operating under atmospheric pressure has proven to be especially economical in view of the energy costs, since in this case the thus-produced vapors can be directly used, without compression, for the preliminary concentration of the solution, which represents a considerable saving in energy.
However, the difficulty has existed as to how to operate such high concentrators under atmospheric pressure, because NaOH and KOH solutions exhibit very high boiling temperatures at atmospheric pressure and are also very corrosive so that the construction materials employed are exposed to extremely high thermal and corrosive stresses.
Due to these relationships, it has not been possible heretofore to operate alkaline melt high concentration devices under practical conditions, since these devices were either operated under vacuum, and the vapor heat could not be recovered, or they were operated under atmospheric pressure, with the simultaneous recovery of the vapor heat, with the disadvantage that the operating temperatures were so high that they adversely affected the durability of the construction materials.
Accordingly, an object of the present invention is to provide a process which utilizes the falling-film principle with an appropriate apparatus for the concentration of an alkaline solution which no longer exhibits the aforedescribed disadvantages.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The process of the present invention is characterized in that the alkaline melt running off from the film support is collected separately from the vapors and further conducted separately from the vapors.
The apparatus for conducting this process, according to the present invention comprises at least one cylindrical evaporator tube equipped with a heating jacket and serving as the falling-film support, wherein an annular collecting trough for the melt is associated with the lower tube edge and is in communication with a discharge line. The interior of the tube is in communication, through a central opening in the collecting trough, with a vapor collecting duct.
This separate collection and discharge of the melt and vapors prevents a remixing of the concentrated melt and the vapors after exiting from the evaporator tubes. This remixing would again considerably raise the water content of the concentrated melt, since the vapors, once they leave the evaporator tube, are not in boiling equilibrium with the melt, i.e., they have a temperature lying below the boiling point of the melt. This is due to the fact that the vapors produced by evaporation in the upper portion of the evaporator tube are not heated up to the boiling temperature of the concentrated melt along their way to the lower edge of the tube.
The immediate separation of the melt and the vapors on the lower edge of the tube, according to the present invention is of economic significance inasmuch as the demanded, high final concentrations can be attained at operating temperatures which are relatively low, even when operating under atmospheric pressure. Therefore, two goals can be achieved according to the present invention, i.e., the reuse of the vapor heat as a consequence of operating at atmospheric pressure and simultaneously, the relatively low operating temperature and the concomitant longer lifetime of the concentrator.
In contrast thereto, an intermixing of the concentrated melt and the vapors takes place in conventional evaporator systems not equipped with a collecting trough for the concentrated melt. This intermixing occurs in the collecting duct, i.e., in the vapor separator and results in a considerable dilution of the concentrated melt until the melt and vapors are again in temperature equilibrium. It has been observed that under these conditions the concentration of the melt can again decrease by almost one-half percent, leading to an unsatisfactory concentration if the melt is to be processed into flakes or prills.
Apart from the obvious advantages attained from the viewpoint of process technique, the collection and removal of the concentrated melt separately from the vapors also have the advantage that the highly corrosive melt does not come into contact with the entire separating system for the melt and vapors, but rather only with those parts used in conducting the melts. These parts require a relatively minor amount of high-quality construction material and can be readily exchanged. However, the voluminous portion of the separator system, namely the vapor discharge duct, can consist of a very thin material since it is no longer in contact with the corrosive melt and thus does not suffer from corrosive deterioration.